Hybridizers: fertilize seedlings

Here i am once again..more questions...I dont think i took this good care of my children when raising them as these seedlings...
Anyway...
My DL seedlings are in plastic cups .Holes in the bottoms.Inside for the winter as i am inMissouri and its cold.Will plant outside in MAY
.Used Miracle Grow Seed Starter mix.Now that it has been a few weeks and they are up to about 4-5 " tall should i fertilize or will the Miracle grow mix carry them for a while ? If yes With what?How often?
Thanks for any help
Roxanne